1. Introduction
This invention relates to novel novolak resins formed from phenols and aromatic aldehydes, to blends of the same with other resins including conventional novolak resins and to a novel process for making said new novolak resins.
2. Description of the Prior Art
The formation of phenolic resins by condensation of a phenol with an aldehyde is well known in the art and described in numerous publications including Kirk-Othmer Encyclopedia of Chemical Technology, Volume 15, pages 176 to 208, 1968, incorporated herein by reference. Phenol itself is the phenol used in the greatest volume for the formation of phenolic resins, but resorcinol, alkyl substituted phenols such as cresols, xylenols, and p-tert-butylphenol and p-phenylphenol are used in substantial volume. The aldehyde used is almost exclusively formaldehyde, but small amounts of acetaldehyde and furfuraldehyde have also been used.
The condensation of a phenol with an aldehyde can be an acid catalyzed reaction with a molar ratio of aldehyde to phenol less than 1 or an alkaline catalyzed reaction with a molar ratio of aldehyde to phenol greater than 1. The acid catalyzed reaction yields thermoplastic resins which have come to be known in the art as the novolak resins while the alkaline catalyzed reactions yield thermosetting resins known as the resole resins.
The novolak resins have many commercial uses. Their primary use is as a molding compound where they are molded to a desired configuration and then cured by crosslinking with a crosslinking agent such as hexamethylene tetramine. Another use of the novolak resins is as a film forming material in a coating composition. For this use, the resin is cast from solution and generally the cast film is not cured.
Conventional novolak resins, prior to cure, have only moderate thermal stability and typically melt within a range of from about 90.degree. C. to 120.degree. C., dependent upon the composition of the resin and its molecular weight. There has been little effort to increase the stability of the thermoplastic novolak resins to high temperatures because high thermal stability has not been considered to be an important property of a film forming resin.
A new class of novolak resins formed by condensation of a mixture of a naphthol and a phenol with an aldehyde is disclosed in U.S. Pat. No. 4,424,315. These resins are copolymers formed by the aforesaid condensation of an aldehyde with an aromatic alcohol mixture of a naphthol and a phenol in the presence of an acid catalyst. The molar ratio of the naphthol to the phenol can vary from about 20 to 1 to 1 to 20 dependent upon the desired properties of the resin. These resins were prepared for use as a binder for a photoresist and show improved resistance to flow at elevated temperatures, though it was found that a photoresist formulated with this resin was difficult to develop.
In Materials for Microlithography, L. F. Thompson, G. G. Wilson, and J. M. Frechet; Eds.; ACS Symposium Series 266, American Chemical Society, Washington, D.C., 1984, Chapter 17, page 339, a meta-cresol-benzaldehyde novolak resin was formulated with a photosensitizer and solvent to produce positive toned images when the mixture was applied to a silicon wafer, exposed to actinic radiation and subsequently developed. However, the synthesis of the cresol-benzaldehyde novolak resin, as taught, produced a material having low molecular weight, and photoresist compositions based on it had low photospeed, low resolution and inadequate temperature resistance.
In U.S. Pat. No. 4,943,511, there is disclosed and claimed a positive photoresist composition which uses a resin binder that is prepared from a phenolic component having a high p-cresol content and an aldehyde that is a mixture of formaldehyde and an aromatic aldehyde. In accordance with the patent, photoresists formulated using the aforesaid resins as binders possess improved resolution capabilities, but it is believed that the resins of the patent exhibit only minimal or no thermal improvement compared to prior art novolak resins.